There isn't a simple "x number of weeks" type answer to this.
It should be straight away, but, of course, most people's conditions aren't consistent every day, and it may take some while before it becomes apparent that there's been a change of circumstances. This can be particularly so if you have a condition that relapses and remits at erratic intervals, with better periods of weeks at a time.
On the other hand, if you have a condition that's fairly consistent and you have a new treatment/surgery that means that there is a difference once it's taken effect, then you'll know when the change has occurred.
Hi Survivor Thanks , I had gone blind in one eye and was awaiting an operation and had to wait 4 mths or so but operation made me permanently blind unfortunately in this eye hence change of circumstance. I did want to wait and see(how ironic) if I could get my sight back again ! But DLA have told me I did not let them know sooner, when I did, in fact write when from on application. The mind boggles